1. Field of the Invention
This invention relates to optical filters and is of particular, but not exclusive, application to passive optical network transmission systems.
2. Related Art
One approach to providing communications links between stations is to employ a passive optical network between a central station, or exchange, and the stations.
A potential strategy for the evolution of passive optical splitter networks providing customer access in the local loop is as follows. Telephony and low data-rate services could initially be provided over a fibre-lean passive optical network (TPON) at a single operating wavelength, such as 1300 nm. Broadband services could then be added in two separate stages. Firstly, coarse-grained wavelength division multiplexing (WDM) could provide a few wavelength channels in the 1300 nm window: 20 nm spacing has been demonstrated, but a closer spacing of perhaps 10 nm is quite feasible. Just three additional wavelengths, 1280 nm, 1320 nm and 1340 nm, would be sufficient to provide a range of early broadband services such as ATM-based high-speed data, 32-channel broadcast digital video and 32-channel dedicated (switched) video. A fourth wavelength might be used to provide several HDTV channels. This approach would leave the 1500 nm window free for high density WDM (HDWDM) to be used in the longer term, when large numbers of channels are needed, in combination perhaps with the use of erbium-doped fibre amplifiers, or to provide two-way broadband customer services by wavelength multiplexing in the upstream direction. A nominal wavelength spacing of 1 nm has been demonstrated for HDWDM.
A major difficulty, however, of employing tunable lasers at customers' premises is that of accurate referencing of the laser wavelengths, particularly for HDWDM, where channels may be spaced as close as 1 nm or less. For broadcast services such as cable TV, tunable filters are key components for selecting wavelength channels.
Many tunable filter technologies have been demonstrated offering various tuning ranges and bandwidths including DFB laser amplifiers, etalons, acousto-optics, and variable-pitch, position-tuned devices based on reflection holograms in dichromated gelatin.
For a device to be suitable for installation in customers' premises, it must employ a technology that lends itself to low cost mass production.